Descriptions

The nature and magnitude of experimental errors due to ⁶⁵Zn
adsorption by inorganic surfaces was examined in a laboratory system
designed to measure ⁶⁵Zn uptake by marine phytoplankton. In the pH
range, 6.3±.1 to 7.5±.1, a precipitate formed in the algal nutrient
medium selected for the system. To this seawater-base medium,
zinc was added only as carrier-free ⁶⁵Zn. The precipitate increased
in both volume and tendency for ⁶⁵Zn uptake as the pH increased. At
a pH of 7.5±.1 the particles, predominantly orthophosphates, accumulated
70 percent of the ⁶⁵Zn in the medium in 24 hours. It was
therefore concluded that ⁶⁵Zn adsorption by undetected precipitates
could result in serious errors in measurements of ⁶⁵Zn uptake by
marine phytoplankton.
Equilibrium of ⁶⁵Zn adsorption by Pyrex glass surfaces was attained
between .50 and 2.0 hours elapsed time. Found to be negligible
at pH values less that 6.0±.1, such uptake was a linear function of
the hydrogen ion concentration in the pH range, 6.7±.1 to 8.2±.1.
Pretreatment of the glass surfaces with dimethyldichiorosilane
(General Electric "Dry Film") reduced ⁶⁵Zn adsorption by over 80
percent.
The relationship between percent ⁶⁵Zn sample adsorption and
wetted glass surface area/pipette sample volume was shown to be
linear for volumetric pipettes of the size range, 1-15 ml. At a pH of
7.5±.1, ⁶⁵Zn adsorption was negligible for most laboratory glassware,
but increased with increased pH. At a pH value of about 8.0,
glassware having surface area/sample volume ratios as small as those
of 15 and 20 ml volumetric pipettes adsorbed 7 to 11 percent of the
contained sample activity. Two prerinses with the sample liquid were
required to reduce such errors by a factor of one half.